1. Field of the Invention
The present invention relates generally to explosives, and specifically to high-energy explosive primer charges. More particularly, the present invention relates to an improved primer charge of cast explosive and method of manufacturing thereof.
2. Description of Related Art
A complete explosive assembly typically comprises a high-explosive main charge, a primer (or booster) charge, and a primary initiator. The primary initiator, typically a blasting cap, electric detonator, or Low Energy Detonating Cord (LEDC), is used to initiate the primer charge. The high-explosive, such as Ammonium Nitrate Fuel Oil (ANFO), is typically insensitive to the primary initiator and requires initiation by the primer charge. Primer charges are typically formed into cylindrical casts, having one or more bores therein for the insertion of the primary initiator. However, bores are not necessary since blasting caps can be taped to the exterior of the explosive charge. Primer charges are also frequently used by themselves as the total explosive charge.
Cylindrical cast primer charges are well known in the art. Various compositions and properties of cast explosive charges are described in, for example, U.S. Pat. No. 3,994,756 issued Nov. 30, 1976, to Hendrickson et al.; U.S. Pat. No. 4,678,524 issued Jul. 7, 1987, to Cranney et al.; U.S. Pat. No. 4,747,892 issued May 31, 1988, to Spencer; and U.S. Pat. No. 4,776,276 issued Oct. 11, 1988, to Yunan, which are hereby incorporated herein by reference.
Primer charges are typically formed into cast compositions from pourable mixtures, such as Pentolite or Cyclotol. Pentolite, a mixture of Pentaerythritol tetranitrate ("PETN") and Trinitrotoluene ("TNT"), is typically mixed in a weight ratio of from 60/40 to 40/60. Cyclotol is a mixture of Cyclonite ("RDX") and TNT, which is typically mixed in weight ratios of from 50/50 to 75/25. Small amounts of other constituents, such as wax, cellulosic resins, metallic particles, or curable plastics may also be utilized in varying proportions to effect desirable physical and chemical properties of the cylindrical cast charge. Other castable explosive compositions include Amatex, Kalatol, Tritonal, Tetrytol, Baratol and Baronal.
The casts are generally prepared by stirring powdered solid components, such as PEIN or RDX (which are essentially insoluble in water and have a higher melting point than TNT), or mixtures thereof, with molten TNT until a homogeneous slurry is obtained. The molten slurry, typically at temperatures exceeding 100.degree.C., is then poured into upright cylindrically-shaped molds or canisters (made of cardboard, plastic, or the like), wherein the molten slurry is allowed to cool and solidify into cylindrical cast primer charges.
Generally, each of the canisters is positioned about a pair of upstanding pins prior to the introduction of the slurry in the canister. After cooling, the entire canister or mold, containing the solidified explosive is pressed off of the pins, leaving the bores extending through the charge.
Present manufacturing processes typically require an individual worker to manually pour the molten mixture into the upright cylindrical canisters. Once the mixture has solidified, the worker must remove the pair of upstanding pins (used to form the bores through the cast primer charge) and tapping the pins out of the canister. The cylindrical primer charge produced by this process is a single-piece casting.
In some manufacturing processes, machines are used to press the solidified explosive off of the pins. In one example, hydraulic presses may be used to press the pins out of the canisters. In some processes, a number of pins may be physically joined together to form a pin bar. Each pin bar may contain several pairs of pins. In this particular approach, several primers are fabricated using each pin bar arrangement.
There are significant safety concerns associated with labor intensive manufacturing facilities, particularly in the production of explosives. Quality control is also difficult to maintain in such environments. Furthermore, the explosive products are becoming increasingly more expensive to produce. This increasing cost reflects the costs of labor, materials, and accompanying problems associated with production. In particular, pentolite and cyclotol are expensive; and although mixtures comprising pentolite and cyclotol are highly desirable from a chemical standpoint, they are often disadvantageous from an economic standpoint.
One of the principal parameters used to compare explosive properties is the rate of detonation ("ROD"), which is the velocity at which the explosive burns. The higher the ROD, the higher the shattering effect of the explosive. Aside from having a higher shattering effect, a higher ROD is desirable from a timing standpoint. For instance, in quarries and open pit mining, it is typically to drill row after row of holes which are charged with explosive and then detonated from front to back in a timed sequence. If one row detonates too fast or too slow the desired effect (uniform stone cleavage or earth removal) will not take place. Thus, it is imperative that the rate of detonation be consistent. Therefore, a consistent higher rate of detonation would lessen or eliminate timing errors.
In commercial explosives, the ROD ranges from approximately 5,000 to 30,000 feet per second (fps). Although the ROD of an explosive charge depends on the density, particle size, and degree of confinement, the ROD of previous cast compositions typically ranges from 27,000 to 30,000 fps. The highest recorded rates of detonation have been in the 34,000 feet per second range.
In view of the foregoing, it would be advantageous to produce a primer charge with a consistent high ROD. It would also be advantageous to have a less-expensive and safer method for manufacturing process for primer charges.